Scientists have produced a thin film out of a new class of metal alloys that can survive very high temperatures and extreme pressures.

Researchers at ETH Zurich then used the film—just 3 micrometers thick—to form “micropillars” that have very special properties: They are 10-times stronger than a block made of the same material.

The pillars can be compressed by up to a third of their length under high pressures without become brittle or cracking. Scientists refer to this deformability as ductility.

A silicon wafer coated with a thin film of the high-entropy alloy. (Credit: Fabio Bergamin / ETH Zurich)A pillar with a diameter of one micrometer before (left) and after (right) high-pressure mechanical deformation. (Credit: Zou et al. Nature Communications 2015)

The high-entropy alloy material—made of equal parts niobium, molybdenum, tantalum, and tungsten—also resists extreme temperatures: It survived three days at 1,100 degrees Celsius with no significant change to its external or internal structure.

That’s in stark contrast to pure tungsten, which the scientists also subjected to heat treatment as a control.

Following heat treatment, micropillars made of the alloy perform significantly better in terms of strength and ductility than those made of pure tungsten. This is despite the fact that the high-entropy alloy’s melting point is significantly lower than that of pure tungsten (around 2,900 versus 3,400 degrees Celsius).

The scientists produced the 3-micrometer film using magnetron sputtering, a coating method often used in the field of microelectronics. This was the first time the technique had been used to produce a high-entropy alloy by atomizing the four elements and spraying them onto a substrate material. Then, using the focused ion beam (FIB) technique, the scientists cut out microcylinders on the surface of the film.

Tiny individual crystals

The material is remarkable not only for its extremely intricate pillar structure but also for its internal crystal structure. Like most crystalline bodies, this material also consists of a large number of small individual crystals. The special feature of the alloy is that these individual crystals are tiny—this is referred to as a nanocrystalline material.

This microscope image shows how the material is made up of individual crystals, with sizes of approximately 30 to 130 nanometers. (Credit: Huan Ma / ScopeM / ETH Zurich)

“Although nanocrystalline materials have many desirable properties, they often also bring disadvantages,” explains Yu Zou, a doctoral student and first author of the study published in the journal Nature Communications. “For example, these materials are usually not temperature-resistant, as heating causes the individual crystals to expand and therefore changes the properties of the material.”

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According to the scientists, the alloy’s ability to withstand extreme temperatures may be related to the relatively disordered atomic distribution of the elements inside the material. In particular, the researchers suspect that the disorder at the internal boundary surfaces of individual crystals in high-entropy alloys means the crystals tend to grow less than in other materials when heated.

Whether this theory is accurate is something the scientists wish to investigate in another research project, in which they will scrutinize the atomic distribution of elements within the material.

Zou says the new material will be of interest above all in high-pressure and high-temperature applications, for example for building sensors that are required to operate in extreme conditions.